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Actions & Reactions

Chemists want to know about matter and its properties—the density, acidity, size and shape of molecules. Biologists want to understand living things and how they interact with their environment.

And chemistry and biology are more connected than you might think.

Using knowledge about atoms, forces, and molecules, chemists learn about unfamiliar substances, but they also learn about organisms and their body processes.

Pathways of Life

Photo of chemist Andy Combs waterboarding
Millions of chemical reactions in the body help us stay active.

One of the most amazing triumphs of the human body is the fact that all we really need to do to keep it running is to eat and sleep. The rest seems to take care of itself. But a lot of chemical reactions are going on 24/7 to make this happen.

Metabolic factories recycle the components of digestion back into basic building blocks from which our tissues and organs are built.

Using proteins, the genetic material DNA and RNA, sugars and fats, chemical reactions let a sprained ankle heal properly, make our hair and fingernails grow a little bit every day and give us energy to text our friends or ace a geometry test.

The main players in metabolism are enzymes, molecules that speed up the chemical reactions in our bodies. Because they make reactions go faster than they would on their own, enzymes are biological catalysts.

Speeding Allowed

Illustration of how a substrate and enyzmes become products
Cascades of enzymes make up metabolism.

Consider this: Without the help of enzymes, the conversion of nutrients and minerals into usable biological molecules such as proteins, DNA and RNA might take weeks, even years! Enzymes are essential to life, since our bodies cannot afford to wait that long to receive the important products of chemical reactions.

In speeding up reactions, enzymes act like the accelerator pedal of a car. But they also play the role of matchmaker, bringing together starting materials (substrates or reactants) and converting them into finished materials (products).

Most enzymes reside inside cells. If cells get damaged, they break apart and spill their contents into neighboring body fluids, like blood. That is why higher-than-normal levels of enzymes in blood—revealed in a simple blood test—can signify trouble in tissues or organs where those cells normally live.

Illustrations of ionic bond, covalent bond and hydrogen bond
Three types of attractive forces hold atoms together to make molecules.

When enzymes are not working properly, they can cause disease. For example, cancer can develop when the enzymes that copy DNA make mistakes. These errors can produce a misspelled gene that makes a defective protein or no protein at all. If that protein is the one that keeps a set of cells from multiplying out of control, you can imagine how its absence could bring about serious problems.

Inside the body, enzymes are never lonely. They link together, forming vibrant networks and pathways. And so our metabolism is really just a collection of enzyme-catalyzed reactions that build and break down organic molecules in food, producing or consuming energy in the process. But to be effective, the reactions need to work together in a coordinated way.

The chemical reactions of metabolism occur over and over again. Much like a cascade of dominoes, the product of one chemical reaction becomes the substrate for another. By understanding the language of the body's metabolic communication systems, scientists can find ways to patch the circuits when they become broken from injury and disease.

Shape Matters

One secret to an enzyme's success is its three-dimensional shape. An enzyme is shaped so that it can hug its substrate tightly. This molecular embrace triggers chemical changes, shuffling attractive forces and producing new molecules. Only enzymes that have an exact fit with their substrates do a decent job of speeding up chemical reactions.

Many proteins need help from one or more other proteins to perform their jobs well. Proteins that interact often change their shape as a result of the encounter. The differently shaped protein is better able to capture its substrate and make a chemical reaction happen. Sort of like rearranging seats in a room to accommodate more guests, the reshaping of proteins can make extra space for substrates and products to fit.

Illustration of proteins working together
Shape changes in proteins help substrates and products fit together.

In addition to proteins, other helper molecules called cofactors are necessary ingredients for many enzyme reactions. Folic acid, a B vitamin, is one of them.

Researchers have known for decades that folic acid can protect against certain birth defects—such as spina bifida—that develop during the first few weeks after conception. For this reason, the U.S. Food and Drug Administration recommends that every woman of childbearing age supplement her diet with 400 micrograms of this vitamin.

Folic acid does its good deeds by improving the fit between various enzymes and their substrates. One of these enzymes speeds up the conversion of a potentially harmful molecule called homocysteine to methionine, a nontoxic amino acid that the body needs. Thus, folic acid lowers blood levels of homocysteine, which in excess is a risk factor for heart attacks and strokes.


This page last reviewed on April 22, 2011